1. Field of the Invention
The present invention relates generally to electric power steering apparatuses equipped with a so-called "variable gear ratio mechanism". More particularly, the present invention concerns an improvement electric power steering apparatus with a variable gear ratio mechanism, which is arranged to impart a sufficient return motion--toward the center or straight-ahead position--of the steering wheel and thereby effectively minimizes the feeling of unsmoothness, uncontinuousness and friction that would be encountered as the vehicle driver turns the steering wheel, without sacrificing advantageous steering-assist characteristics, and which thus achieves an improved steering feel and stable vehicle behavior during both low-speed and high-speed driving.
2. Description of the Related Art
Electric power steering apparatuses have been known which, using a variable gear ratio mechanism, such as a worm sector, ball nut or rack-and-pinion type, are designed to set a larger gear ratio when the steering wheel is at or near the straight-ahead position and set a smaller gear ratio when the steering wheel is at or near the full lock position, so as to provide an enhanced vehicle response. Also, electric power steering apparatuses have been commonly known which control an electric assisting motor to reduce its electric steering assist as the vehicle travels faster. Such known electric power steering apparatuses are arranged to minimize undesirable variations in the vehicle's behavior, by reducing the rotational speed setting of the motor during a driver's return steering operation in accordance with an increase in the vehicle velocity; that is, the returning speed of the vehicle's steering system is kept relatively high during low-speed driving and relatively low during high-speed driving (slow response during low-speed driving and quick response during high-speed driving).
Further, the known electric power steering apparatuses equipped with a variable gear ratio mechanism are constructed to avoid an increase in the number of steering wheel revolutions by setting a small overall steering ratio, because a relatively great overall steering ratio would lead to an increased number of steering wheel revolutions even though it does reduce steering effort by the vehicle driver.
In addition, because of the above-mentioned arrangement that a larger gear ratio is set when the steering wheel is at or near the straight-ahead position while a smaller gear ratio is set when the steering wheel is at or near the full lock position, i.e., that the gear ratio is decreased as the steering angle increases, the steering wheel movement is given some operational margin to prevent too quick a directional change of the steered wheels during straight-ahead vehicle travel while reducing the number of lock-to-lock revolutions of the motor. However, this arrangement would result in the following disadvantages.
First, because influences of mechanical friction, viscosity and inertia of oil, etc., appearing in upstream regions, from the variable gear ratio mechanism up to the steering wheel, of the steering system, tend to differ depending on the gear ratio selected, the returning speed of the steering wheel would also vary in accordance with the selected gear ratio irrespective of self-returning torque from the vehicle wheels. This inconvenience would be serious particularly during low-speed driving because the self-returning torque is smaller than during high-speed driving. Second, even when a steering assist is applied to outward (away from the center position toward one of the full lock positions) and return motions of the steering wheel by positively utilizing output power from the electric motor, it is difficult to impart a suitable steering assist due to gear ratio changes by the variable gear ratio mechanism; this would unavoidably involve the feeling of unsmoothness, uncontinuousness and friction, thereby resulting in a deteriorated steering feel.